Update README and Dockerfile to include llvm-proto-fuzzer

Summary: Added commands to Dockerfile to build llvm-proto-fuzzer and the other related tools. Also added a section to the bottom of the README describing what llvm-proto-fuzzer does and how to run it.

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D50829